sub walk_tree
{
  my $tree = shift;
  my $per_node = shift;
  walk_tree($tree->left);
  $per_node->($tree->node);
  walk_tree($tree->right);
}

sub print_tree
{
  my $tree = shift;
  walk_tree($tree, sub { print $_[0]; });
}

sub sum_tree
{
  my $tree = shift;
  my $total = 0;
  walk_tree($tree, sub { $total += $_[0]; });
  return $total;
}